Optical transceivers are commonly used in optical networking for conversion between an optical signal and an electrical signal. In general, an optical transceiver includes a Transmit Optical Subassembly (TOSA), which typically includes a light-emitting device such as a laser chip, and a Receiver Optical Subassembly (ROSA), which typically includes a light-receiving device such as photodiode.
Referring to FIG. 1, a typical TOSA includes a spot size converter 120, an isolator 140, and a beam splitter 160. The spot size converter 120 shapes and/or sizes the beam of light emitted from the laser chip 101 and collimated by the collimating lens 115 to improve coupling efficiency into an optical fiber, and thus provide higher power. The isolator 140 prevents back-reflection light from reaching the laser chip 101, while allowing the light to pass to the beam splitter 160. The beam splitter 160 allows a portion of the light to be tapped and diverted to a power/wavelength monitor 170, while allowing the remaining light to be transmitted to the optical fiber coupled to the fiber pigtail 190 via the focusing lens 175.
Conventionally, the spot size converter 120, isolator 140, and beam splitter 160 in a TOSA are individual optical components, which function independently from the others.
The spot size converter 120 shapes and/or sizes the beam of light emitted from the laser chip (e.g., a laser diode). In particular, since the output beam of light of most laser diodes has an elliptical cross-section (e.g., elongated in the vertical direction), the spot size converter will often reduce the beam aspect ratio (e.g. beam waist ratio along horizontal and vertical axes) such that the cross-section of the beam of light is closer to circular. Conventionally, spot size converters are waveguide-based, and thus can be integrated on the laser chip. For example, one example of waveguide-based spot size converter is described in U.S. Pat No. 7,664,352. In general, waveguide-based spot-size converters include waveguides sections having tapered widths or thicknesses. Since waveguide-based spot-size converters are used to match the mode of the output waveguide of the laser chip 101 to the mode of larger optical fiber coupled to the fiber pigtail 190 they are often referred to as mode transformers. Unfortunately, since waveguide-based spot-size converters are integrated with the laser chip, the waveguide structure is more complex, with a more complicated fabrication process. In addition, these integrated waveguide structures create extra light loss due to transmission mode mismatch. Other approaches used to shape and/or size a beam emitted from a laser include using collimating/focusing lenses or a pair of anamorphic prisms. Unfortunately, these approaches result in a relatively bulky spot-size converter.
The isolator 140 is an optical isolator, which passes the forward propagating light advancing from the laser chip 101 and prevents the backward propagating light from propagating to the laser chip 101 (e.g., back reflected light). In general, the optical isolator 140 may be a polarization dependent isolator or a polarization independent isolator.
A polarization dependent isolator typically includes an input polarizer, a Faraday rotator, and an output polarizer (i.e., often referred to as an analyzer). Both the input polarizer and analyzer are absorptive polarizers (e.g., an absorptive film polarizer), which absorb the unwanted polarization states. Referring to FIG. 2a, which shows one embodiment of a polarization dependent isolator, the input polarizer 242a is polarized vertically, whereas the analyzer 246a is polarized at 45°. In the forward propagating direction, the light passes through the input polarizer 242a and becomes polarized vertically, passes through the Faraday rotator 244a (e.g., latched garnet) wherein the polarization is rotated by 45°, and passes through the analyzer 246a. In the backward propagation direction, the light passes through the analyzer 246a and becomes polarized at 45°, passes through the Faraday rotator 244a which again rotates the polarization by 45°, and is blocked by the input polarizer 242a (i.e., since the light is polarized horizontally, but the input polarizer only passes light polarized vertically).
A polarization independent isolator typically includes an input birefringent wedge, a Faraday rotator, and an output birefringent wedge. Both the input and output birefringent wedges are beam-splitting polarizers, wherein an unpolarized beam is split into two beams with opposite polarization states. Referring to FIG. 2b, which shows one embodiment of a polarization independent isolator, the input birefringent wedge 242b has its ordinary polarization direction vertical and its extraordinary polarization direction horizontal, whereas the output birefringent wedge 246b has its ordinary polarization direction at 45°, and its extraordinary polarization direction at −45°. In the forward propagating direction, the light passes through the input birefringent wedge 242b and is split into its vertical (o-ray) and horizontal (e-ray) components, passes through the Faraday rotator 244b which rotates both the o-ray and e-ray by 45° such that the o-ray is at 45° and the e-ray is at −45°, and is recombined by the output birefringent wedge 246b. In the backward propagating direction (not shown), the light is split into the o-ray (at 45°) and the e-ray (at −45°) components by the second birefringent wedge 246b, passes through the Faraday Rotator 244b, which rotates both rays by 45°, and passes through the first birefringent wedge 242b, which causes the two rays to diverge such that the two rays are vertically offset from the forward propagating input ray, and thus prevented from propagating to the input port. In general, the light beam is incident on the oblique surface of the first birefringent wedge at essentially the wedge angle. Notably, although the use of beam-splitting polarizers advantageously provides a polarization independent isolator, the beam-splitting polarizers are more bulky, expensive, and complicated than absorptive polarizers.
The beam splitter 160 allows some of the light being transmitted to the optical fiber connected to the fiber pigtail 190 to be diverted, and thus allows power/wavelength monitoring. Referring to FIG. 3, one example of a commonly used beam splitter 160 in TOSAs is a plate beam splitter. Plate beam splitters often include a thin film reflection coating (e.g., partially reflective coating) disposed on a surface of a glass plate, which is positioned at 45° angle of incidence, so that a portion of the light striking the plate beam-splitter is reflected and another portion is transmitted. The reflected portion is directed towards the power/wavelength monitor.
As discussed above, the spot size converter 120, isolator 140, and beam splitter 160 conventionally are provided as individual parts/components, which are packaged together in a TOSA package. Notably, the use of three different parts results in a relatively high material cost, makes the package bulky (e.g., due to spacing of parts), and makes the assembly of the parts relatively complicated.
In U.S. Pat. No. 6,330,117, Seo combines isolator and beam splitter functions together in a single component. Referring to FIG. 4, the integrated component includes a first polarizer 442 (i.e., a wedge shaped beam-splitting polarizer), a Faraday rotator 444, and a second polarizer 446 (i.e., a wedge shaped beam-splitting polarizer). The first polarizer 442 has a partial reflective coating disposed on a front surface thereof, and thus functions as a beam splitter. In operation, a collimated beam of light is incident on the first polarizer 442, wherein a portion of the light is reflected and directed to a power monitor, while the remaining portion is transmitted to the Faraday rotator 444 and the second polarizer 446. Advantageously, providing the partial reflective coating on a surface of the wedge shaped input polarizer 442 provides a simpler structure and obviates the need for a separate plate beam splitter. Unfortunately, since the isolator uses two wedge-shaped birefringent crystals, the resulting component is relatively bulky.